The purpose of this project was to make a comparative analysis of tyrosine hydroxylase in rat pineals from light-adapted and dark-adapted rats. In this gland both the rate of utilization and concentration of N.E. increased when animals are in the dark phase of a 24 hr light/dark cycle. Thus, a comparative analysis would reveal alterations in tyrosine hydroxylase activity which could possibly account for the observed increase in neurotransmitter synthesis. Using subsaturation cofactor concentrations T.H. activity in dark-adapted rats was 60 percent greater than in appropriate light control animals. Further kinetic analysis showed that the Km of cofactor using tissue from dark-adapted animals was 0.8 plus or minus 0.1 mM which was significantly lower than a Km value of 1.4 plus or minus 0.2 for light-adapted animals. Thus, the rate of norepinephrine synthesis in vivo could increase due to an alteration in the enzyme molecule.